This application relates to and incorporates herein by reference Japanese Patent Application No. 2001-263394 filed Aug. 31, 2001.
The present invention relates to an electric power supply system for engine starters, that uses a battery and a capacitor.
An engine starter motor requires a large starting current when driven to start a vehicle engine. This large starting current lowers the voltage of a battery in a vehicle, necessitating prolongation of period of driving the starter motor. In a vehicle having an economy-running system (idle-stopping system) which automatically stops engine operation at traffic lights or traffic congestion for fuel economy and reduced exhaust emission, the decrease in the battery voltage when driving the starter motor for engine restarting also adversely affects electric accessory devices such as a navigation device and an audio device in the vehicle.
It is proposed to provide an electric power source (capacitor) exclusively for the starter motor in addition to the battery or a back-up power source (capacitor) for each electric device in the vehicle. However, a plurality of capacitors needs to be provided for each of the starter and the electric devices, because each capacitor has only limited capacitance. As a result, the electric power source must be sized large and a control unit for charging each capacitor evenly becomes complicated, resulting in increase in cost.
It is therefore an object of the present invention to provide an electric power supply system for engine starters that compensate for a decrease in battery voltage by a small capacitor.
According to the present invention, an electric power supply system for a starter comprises a capacitor. This capacitor is charged by a battery through a charging circuit including resistors that charges the capacitor by dividing a voltage of the battery by the resistors. This capacitor is discharged to the starter through a discharge circuit that applies to the starter a charged voltage of the capacitor in addition to the voltage of the battery. When the starter is driven, the connection of the capacitor is switched from the charging circuit to the discharging circuit.
Preferably, the charging circuit includes a series resistor that is connected to the battery in series with the capacitor to limit a charging current to the capacitor, and a parallel resistor that is connected in parallel with the capacitor and in series with the series resistor to divide the voltage of the battery applied to the capacitor. The resistors have respective resistances that define a voltage dividing ratio which provides a capacitor voltage that compensates for a decrease in the voltage of the battery at the time of starting an engine.